1. Field of the Invention
The present invention relates to an image processing apparatus that scans the front and rear surfaces of a document and detects whether the scanned document is a blank sheet, an image forming apparatus having the image processing apparatus, an image processing method performed by the image processing apparatus, an image processing program that causes a computer program to perform the image processing method, and a recording medium having the image processing program recorded thereon.
2. Description of the Related Art
Up until now, known image processing apparatuses scan a document image and perform a blank-sheet detection process based on the characteristics of the data of the scanned image. As examples of the image processing apparatuses of this type, the inventions described in Patent Documents 1 through 3 are known.
Patent Document 1 describes a technique that is capable of acquiring image data according to plural image conditions (such as resolution) and that performs a blank-sheet detection process at low resolution, scans a document at high resolution when the document is not a blank sheet, and accumulates image data. Specifically, a scanner has an image memory that stores the image data scanned from the document by an image sensor, an image processing unit that applies image processing to the image data, and a transmission unit that transmits the image data to external devices. The image processing unit is arranged in a path through which the image data are transferred from the image memory to the transmission unit, making it possible to change the content of an image processed by the image processing unit. The external devices acquire and analyze first image data processed by the scanner according to a first image mode and has an image analysis unit that detects the status (e.g., blank or nonblank) of the document of which the first image data have been scanned. Based on a detection result of the image analysis unit, the external devices acquire second image data processed by the scanner according to a second image mode.
Patent Document 2 refers to a case in which double-sided/single-sided documents are mixed together and describes a technique that faithfully outputs the documents when the documents are output in the form of a sheet or that skips a blank sheet when the documents are saved in an electronic file in one scanning. Specifically, a blank-sheet detection unit performs a blank-sheet detection process with respect to scanned data, a blank sheet page processing unit deletes the scanned data of a page detected to be a blank sheet, and an electronic file conversion unit converts the scanned data of a page detected not to be a blank sheet into an electronic file and then bundles the pages converted into the electronic files. A file management module stores the electronic files in a file storage module. A print image conversion unit converts the scanned data into print images. A print image account unit calculates the accounts of the print images. The print images are transmitted to a printer by a print image transmission module.
Patent Document 3 describes a technique that changes blank-sheet detection parameters according to resolution. Specifically, thresholds adapted to correspond to line densities are stored in a RAM. A line density determination unit determines a set line density in scanning a document, and a threshold selection unit and a setting unit set the threshold corresponding to the line density to detect whether the document is a blank sheet. The thresholds adapted to correspond to the line densities stored in the RAM are automatically changed based on instructions from the user or the number of black pixels scanned from a document having a large white area which is to be used as an ordinal document.
Patent Document 1: JP-A-2005-109754
Patent Document 2: JP-A-2005-354591
Patent Document 3: JP-A-2001-143050
As described above, the known image processing apparatuses scan the document image and perform the blank-sheet detection process based on the characteristics of the image data. In determining the status of the document, the image processing apparatuses of this type perform binarization processing based on a certain threshold and make a count of how many black pixels exist in the document to find out whether the counted number of black pixels exceeds a certain number. However, in case the document is a double-sided document having information printed on both its front and rear surfaces, the number of black pixels may not be detected due to offset from the surface on the opposite side when the number of black pixels of the surface on one side is counted. Furthermore, it is necessary to change the parameters for detecting a blank sheet such as binarized thresholds and the number of counted black pixels when image data from scanners having different scanning characteristics are received at the same time.